On Sat, 15 Dec 2007 11:15:12 +0000, Mike Williams
<nospam@[EMAIL PROTECTED]
> declaimed the following in
soc.genealogy.computing:
> Before 1 May 2000, the non-military GPS signals contained a random error
> of up to 10 metres anyway, so that would swamp any error due to a datum
Selective availability (SA) dither was up to 100 meters at the
extreme. Current non-SA operation gives 10m with the C/A signal used by
civilian gear; 3-5m with WAAS enabled receivers (in CONUS; EGNOS in
Europe. Outside of those areas one should turn off the WAAS/EGNOS mode).
While GPS uses WGS-84 internally, if one is referencing a paper map,
one should set the GPS to the datum printed on the map. Typically NAD-27
for USGS, OSGB for UK (confusingly, OSGB is both a datum AND a
coordinate notation, and the two can be set independently of each
other). Some USGS maps /may/ have correction tick marks in the corners
taking a NAD-27 map up to NAD-74 or similar, but one would have to
hand-draw the newer grid onto the map.
For my apartment, WGS84/UTM gives:
10 S 0585167
4133002
using NAD-27 CONUS (the unit has something like 10 NAD-27 to cover Cuba,
Mexico, Canada, Alaska...)/UTM:
10 S 0585261
4132809
So if one were to take a "native" WGS84 fix from a GPS, and plot my
apartment on a NAD-27 USGS topo map, one would be off by 94m to the
west, and 193m to the north... 214.67m off (704 feet, over a tenth of a
mile)... That would take you from my apartment complex into the Safeway
two streets away!
--
Wulfraed Dennis Lee Bieber KD6MOG
wlfraed@[EMAIL PROTECTED]
(Bestiaria Sup****t Staff: web-asst@[EMAIL PROTECTED]
)
HTTP://www.bestiaria.com/
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